Saute the mushrooms in the oil. Add the flour and stir constantly over medium-low heat for 2 minutes to brown the flour for more flavor. Add the remaining ingredients and stir with a wire whisk until the sauce is well mixed and there are no lumps. Stir constantly over medium heat until very hot. Reduce the heat and cook on the lowest setting for 15 minutes until the sauce is thickened. Stir occasionally while simmering. Serve over hot vegetables.